Sleep Calculator
Optimal21:45
Suggested Bedtimes (6 cycles)21:45
Suggested Bedtimes (5 cycles)23:15
Suggested Bedtimes (4 cycles)00:45
Suggested Bedtimes (3 cycles)02:15
This sleep calculator works backward (or forward) through 90-minute sleep cycles so you wake at the end of a cycle rather than in the middle of deep sleep. Waking between cycles is what leaves you groggy, so the tool suggests bedtimes for a target wake time, or wake times for a chosen bedtime, allowing 15 minutes to actually fall asleep. Each suggestion corresponds to a whole number of complete cycles.
Formula
offset = cycles × 90 min + 15 min (fall-asleep buffer)
- cycles
- Number of complete 90-minute sleep cycles (3 to 6)
- 90 min
- Length of one full sleep cycle
- 15 min
- Time allowed to fall asleep before cycle 1 starts
- offset
- Total minutes subtracted from a wake time, or added to a bedtime
How it works
- Decide whether you know your wake-up time (and need bedtimes) or your bedtime (and need wake times).
- Enter that time in 24-hour HH:MM form. The calculator assumes 15 minutes to fall asleep before the first cycle begins.
- It steps in 90-minute cycles to offer four options. For a wake time it suggests bedtimes for 6, 5, 4, and 3 cycles; for a bedtime it suggests wake times after 3, 4, 5, and 6 cycles.
Worked example
You must wake at 07:00 and want bedtime options.
- 6 cycles: 6 × 90 + 15 = 555 min before 07:00 → 21:45.
- 5 cycles: 5 × 90 + 15 = 465 min → 23:15.
- 4 cycles: 4 × 90 + 15 = 375 min → 00:45.
- 3 cycles: 3 × 90 + 15 = 285 min → 02:15.
Best bedtimes for a 07:00 wake-up: 21:45, 23:15, 00:45, or 02:15 (21:45 gives a full six cycles).
Frequently asked questions
- Why are sleep cycles 90 minutes long?
- A typical adult sleep cycle progresses through light, deep, and REM stages over roughly 90 minutes. Timing your sleep to whole cycles aims to wake you during light sleep, when it is easiest to feel rested.
- Why does the calculator add 15 minutes?
- Almost nobody falls asleep instantly. The tool adds a 15-minute buffer so the cycle count starts from when you are actually asleep, not the moment your head hits the pillow.
- How many cycles should I aim for?
- Most adults do well on five or six cycles, which is about 7.5 to 9 hours of sleep. Fewer cycles can work occasionally, but consistently aiming for five or six supports better rest.
- Is waking at the end of a cycle guaranteed to feel better?
- It is a helpful guideline, not a guarantee. Cycle length varies from person to person and night to night, so treat the suggested times as targets to experiment with rather than exact rules.